This invention generally relates to neutron detectors, and more specifically, to neutron detectors using a neutron sensitive gas.
Neutron detectors are important in many applications including, among others, security, neutron measurements in industrial applications, and neutron physics. Neutrons do not have any electrical charge and thus are capable of passing through thick, protective shielding or concealing materials, and because of this, neutron detectors are used to detect concealed nuclear materials. At the same time, as neutrons have no charge, neutrons may not interact directly with electronic sensing devices, and as a result, neutrons may be very difficult to detect.
Until the 1980's BF3 proportional counters were the standard in neutron detection. Although BF3 proportional counters have been manufactured and used continuously for over 50 years, He-3 proportional counters were chosen for some applications because the gas was made available and because He-3 can be used at higher pressures. He-3, however, is a scarce material and it does not occur naturally. This makes the availability and future supply of He-3 detectors uncertain.
In a He-3 proportional counter, a neutron reacts with He-3 to produce a triton and a proton. These reaction products deposit their kinetic energy in the gas, creating ion pairs which separate in the electric field established in the detector where the electrons gain sufficient kinetic energy to ionize other gas molecules, thereby amplifying the electric signal, which results in a measurable current pulse at the output of the detector.
BF3 proportional counters operate according to the same principles as He-3 proportional counters, except that the neutron sensitive material is the 10B contained in the gas. Boron is a very good thermal neutron absorber due to the high absorption cross section of 10B, which has 19.8% abundance in the natural boron. Enriched Boron is readily available to increase the absorption probability further.
The thermal neutron interaction with 10B is the (n,α) 7Li reaction shown below.
                             10            ⁢      B        +                           1            ⁢      n        ->                                                                       7                        ⁢            Li                    +                                                   4                        ⁢            α                                                Q          =                      2.792            ⁢                                                  ⁢                          MeV              ⁡                              (                                  ground                  ⁢                                                                          ⁢                  state                                )                                      ⁢            6            ⁢            %                                                                                                 7                        ⁢            Li                    +                                                   4                        ⁢            α                                                Q          =                      2.310            ⁢                                                  ⁢                          MeV              ⁡                              (                                  excited                  ⁢                                                                          ⁢                  state                                )                                      ⁢            94            ⁢            %                              
This interaction will release a total energy of 2.792 MeV with the reaction product 7Li in the ground state or 2.310 MeV with the 7Li in the excited state. The latter reaction will happen 94% of the time. 7Li in the excited state will immediately decay to the ground state and release a gamma ray with energy of 0.48 MeV.
The large amount of energy released in the above reaction is shared by the 7Li and alpha particles and can ionize matter and generate electronic signals in a detector. The most common use of the 10B neutron reaction for neutron detection is in the BF3 gas-proportional counter or a proportional counter with 10B lined walls. The advantage of using BF3 over 10B lined walls is the greater intrinsic sensitivity of the detectors and the simplicity of the design. In order to approach the sensitivity of a BF3 counter, 10B lined tubes must employ complex geometries to increase the surface area of 10B inside the tube volume or use many closely packed small diameter tubes or straws. This increases manufacturing costs and introduces unnecessary complexity into the design, which leads to a higher probability of failure.